Sci 142 Chapter 12 Ocean Floor

The Ocean Floor 

Chapter 12

The vast world ocean 

!Earth is often referred to as the blue planet 

• Seventy-one percent of Earth’s surface is 

represented by oceans and marginal seas 

• Continents and islands comprise the remaining 

29% 

!Northern Hemisphere is called the land 

hemisphere, and the Southern Hemisphere 

the water hemisphere

The vast world ocean 

!Four main ocean basins 

• Pacific Ocean - the largest and has the greatest 

depth 

• Atlantic Ocean – about half the size of the 

Pacific and not quite as deep 

• Indian Ocean – slightly smaller than the 

Atlantic, largely a southern Hemisphere body 

• Arctic Ocean – about 7 percent the size of the 

Pacific 

• Land vs ocean average elevation 

• Land – 840 m 

• Ocean – 3729 m

Mapping the ocean floor 

!Bathymetry – measurement of ocean depths 

and the charting of the shape or topography 

of the ocean floor 

!Echo sounder (also referred to as sonar) 

• Invented in the 1920s 

• Primary instrument for measuring depth 

• Reflects sound from ocean floor


!Multibeam sonar 

• Employs and array of sound sources and 

listening devices 

• Obtains a profile of a narrow strip of seafloor 

!Measuring the shape of the ocean surface 

from space


!Three major topographic units of the ocean 

floor 

• Continental margins 

• Ocean basin floor 

• Mid-ocean ridge

Continental margins 

!Passive continental margins 

• Found along most coastal area that surround the 

Atlantic Ocean 

• Not associated with plate boundaries 

• Experience little volcanism and 

• Few earthquakes



• Features comprising a passive continental 

margin 

• Continental shelf 

• Flooded extension of the continent 

• Varies greatly in width 

• Gently sloping 

• Contains oil and important mineral deposits




margin 

• Continental shelf 

• Some areas are mantled by extensive glacial 

deposits 

• Most consist of thick accumulations of shallowwater 

sediments




margin 

• Continental slope 

• Marks the seaward edge of the continental shelf 

• Relatively steep structure 

• Boundary between continental crust and oceanic 

crust




margin 

• Submarine canyons and turbidity currents 

• Submarine canyons 

• Deep, steep-sided valleys cut into the 

continental slope 

• Some are seaward extensions of river valleys 

• Most appear to have been eroded by 

turbidity currents




margin 

• Submarine canyons and turbidity currents 

• Turbidity currents 

• Downslope movements of dense, sedimentladen 

water 

• Deposits are called turbidites




margin 

• Continental rise 

• Found in regions where trenches are absent 

• Continental slope merges into a more gradual 

incline – the continental rise 

• Thick accumulation of sediment 

• At the base of the continental slope turbidity 

currents that follow submarine canyons deposit 

sediment that forms deep-sea fans


!Active continental margins 

• Continental slope may descend abruptly into a 

deep-ocean trench 

• Located primarily around the Pacific Ocean 

• Accumulations of deformed sediment and 

scraps of ocean crust form accretionary wedges 

• Some subduction zones have little or no 

accumulation of sediments

Ocean basin floor 

!Deep-ocean trenches 

• Long, relatively narrow features 

• Deepest parts of ocean 

• Most are located in the Pacific Ocean 

• Sites where moving lithospheric plates plunge 

into the mantle 

• Associated with volcanic activity 

• Volcanic islands arcs 

• Continental volcanic arcs


!Abyssal plains 

• Likely the most level places on Earth 

• Sites of thick accumulations of sediment 

• Found in all oceans 

!Seamounts and guyots 

• Isolated volcanic peaks 

• Many form near oceanic ridges


!Seamounts and guyots 

• May emerge as an island 

• May sink and form flat-topped seamounts 

called guyots or tablemounts 

!Mid-ocean ridge 

• Characterized by 

• An elevated position 

• Extensive faulting 

• Numerous volcanic structures that have developed 

on newly formed crust



• Interconnected ridge system is the longest 

topographic feature on Earth’s surface 

• Over 70,000 kilometers (43,000 miles) in length 

• Twenty-three percent of Earth’s surface 

• Winds through all major oceans 

• Along the axis of some segments are deep 

downfaulted structures called rift valleys



• Consist of layer upon layer of basaltic rocks 

that have been faulted and uplifted 

• Mid-Atlantic Ridge has been studied more 

thoroughly than any other ridge system 

• animation

Seafloor sediments 

!Ocean floor is mantled with sediment – 

causes flatness of abyssal plain 

!Sources 

• Turbidity currents 

• Sediment that slowly settles to the bottom from 

above 

!Thickness varies 

• Thickest in trenches – accumulations may 

approach 10 kilometers


!Thickness varies 

• Pacific Ocean – about 600 meters or less 

• Atlantic Ocean – from 500 to 1000 meters thick 

!Mud is the most common sediment on the 

deep-ocean floor


!Types of seafloor sediments 

• Terrigenous sediment 

• Material weathered from continental rocks 

• Virtually every part of the ocean receives some 

• Fine particles remain suspended for a long time 

• Oxidation often produces red and brown colored 

sediments



• Biogenous sediment 

• Shells and skeletons of marine animals and plants 

• Most common are calcareous oozes produced from 

microscopic organisms that inhabit warm surface 

waters 

• Siliceous oozes composed of skeletons of diatoms 

and radiolarians 

• Phosphate rich materials derived from the bones, 

teeth, and scales of fish and other marine organisms

Diatoms, Radiolarians, Foraminiferans



• Hydrogenous sediment 

• Minerals that crystallize directly from seawater 

• Most common types include 

• Manganese nodules 

• Calcium carbonates 

• Metal sulfides (Fe, Ni, Zn, Ag, others) 

• Evaporites – halite, gypsum


!Distribution 

• Coarse terrigenous deposits dominate 

continental margin areas 

• Fine-grained terrigenous material is common in 

deeper areas of the ocean basin 

• Hydrogenous sediment comprises only a small 

portion of deposits in the ocean 

• There are a few places where very little 

sediment accumulates (Mid-ocean ridges) 

• Coral Atolls

Resources from the seafloor 

!Energy resources 

• Oil and gas 

• Gas hydrates 

!Other resources 

• Sand and gravel 

• Evaporative salts 

• Manganese nodules – contain Mn, Fe, Cu, Ni, 

other metals

Sci 142 Chapter 12 Ocean Floor

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